This invention relates to a digital device for computing a rate of change of an input signal and particularly to circuit means for noise spike clipping in a given direction to discern a unidirectional transient in the input signal.
Digital devices for measuring the rate of change of an input signal are well known in the art. Typically, the input signal is summed with one or more compute cycle delay of the input so that the output of the transient signal is indicative of the rate of change in either polarity. In application, say for a surge control for a gas turbine engine, it may be allowable to sense in only a given polarity and hence it may be possible to discriminate the input signal for only a unidirectional transient.
We have found that we can discriminate a unidirectional transient in the input signal over a two compute cycle period and provide a noise spike clipping circuit. This invention contemplates selecting the maximum of an input transient signal which is a function of the variable being sensed at a given time and the summed value of a function of the variable at one compute cycle delay and a programmed constant. The input signal as first measured can be less than the value of the delayed signal by at most the value of the constant. Hence, the negative spike is clipped by the amount selected by the value of the constant. By employing clipping, lower threshold values for control system action or faster filter time constants can be realized while diminishing the potential false indication due to noise spikes.
The device has been found to have particular utility in a surge control for a gas turbine engine although it should be understood that the invention has utility in any other application where a unidirectional transient in the input signal is desired. As for example in U.S. application Ser. No. 762,763 filed on Jan. 26, 1977 be E. Preti and H. W. Ripy and assigned to the same assignee, a surge control that responds to the temperature at the inlet of the engine is disclosed. In that particular installation surge can de detected by a sudden rise in engine inlet temperature, but since immediate corrective action is required because surge has been initiated, false detection can be hazardous, and hence cannot be tolerated. Hence, maximum sensitivity and response of the detection circuit is desired to assure that recovery can be achieved before surge has been allowed to become destructive.
In actual tests, a surge detection system employing this invention has proven to be efficacious and no false surge detections were recorded. The output transient from the computed temperature signal triggers the surge recovery logic of the control at a positive value of 1.25.degree. F. per compute cycle after being filtered. The spike clipping circuit reduces downspikes to a maximum of 2.degree. F. per compute cycle. Obviously the spike reduction allows a faster filter time constant to be used in the transient filter hence increasing response time of the detection circuit and allowing surge recovery to be initiated sooner.